Earth moving equipment such as motor graders, bulldozers and the like typically have a controllable implement that moves the earth. Likewise, construction equipment, like a cold planer, used in connection with resurfacing roadways, will have an implement that contacts the ground. The implement is manually controlled by an operator who manipulates a series of levers or other devices to effect the desired motion. Many tasks require a high level of skill and demand high levels of operator attention to perform. For example, when manually grading a surface, the operator must pay close attention to the height of the implement to make sure that it stays relatively constant with respect to the surface to thereby prevent the implement from digging too deeply and gouging the surface or raising too high and leaving a bump in the graded surface. While an experienced operator can generally perform this task, inexperienced operators or fatigued experienced operators are much less efficient.
To overcome this problem, various devices have been developed to automatically control the height of the implement. For example, one such system employs a datum, typically in the form of string grade line that is placed at a predetermined height above the desired grade. A sensing unit is attached to the implement and includes a wand that extends from the sensing unit and rests on the string grade line. As the implement height changes, the wand's relation to the grade line changes, causing the sensing unit to generate an error signal. The error signal is then used by the control system to automatically adjust the height of the implement. One drawback to the use of the wand system is that if the spring force of the wand is excessive, or if the string grade line is loosely strung, then the wand can displace the string from its desired position, thereby introducing an error in the control of the implement. Another drawback is that the wand must be on the string grade line to operate properly. To start automatically operating the implement, the operator must get out of the machine and set the wand on the string line. If the wand falls off of the string line, the sensing unit will produce a large error signal which will cause the implement to gouge the earth surface.
An automatic control system that uses an ultrasonic range finding device, has been developed to overcome some of these problems associated with prior art automatic control systems. For example, one such system is disclosed in U.S. Pat. No. 4,733,355 issued to Davidson et al. In that device, an ultrasonic range finding device is mounted to the implement and measures a separation distance from the range finding device to the earth's surface. A comparison circuit periodically compares the measured separation distance to a first, second, and third predetermined intervals and produces a positive error signal if the separation distance is within the limits of the first comparison interval, generates no error signal if the separation distance is within the second comparison interval and generates a negative error signal if the separation distance is within the third comparison interval. One drawback with the system disclosed in Davidson et al. is that it will not work outside the predetermined comparison intervals. Thus, the operator must manually move the implement to a position within one of the three comparison intervals before the system will automatically position the implement.
It would be preferable to have a control system that overcomes these and other drawbacks associated with the prior art.